Multi-field/-scale interactions of turbulence with neoclassical tearing mode magnetic islands in the DIII-D tokamak

Abstract

We present the first localized measurements of long and intermediate wavelength turbulent density fluctuations (ñ) and long wavelength turbulent electron temperature fluctuations (T̃e) modified by m/n=2/1 Neoclassical Tearing Mode (NTM) islands (m and n are the poloidal and toroidal mode numbers, respectively). These long and intermediate wavelengths correspond to the expected Ion Temperature Gradient and Trapped Electron Mode scales, respectively. Two regimes have been observed when tracking ñ during NTM evolution: (1) small islands are characterized by a steep Te radial profile and turbulence levels comparable to those of the background; (2) large islands have a flat Te profile and reduced turbulence level at the O-point. Radially outside the large island, the Te profile is steeper and the turbulence level increased compared to the no or small island case. Reduced turbulence at the O-point compared to the X-point leads to a 15% modulation of ñ2 across the island that is nearly in phase with the Te modulation. Qualitative comparisons to the GENE non-linear gyrokinetic code are promising with GENE replicating the observed scaling of turbulence modification with island size. These results are significant as they allow the validation of gyrokinetic simulations modeling the interaction of these multi-scale phenomena.

Hysteresis Relation between Turbulence and Temperature Modulation during the Heat Pulse Propagation into a Magnetic Island in DIII-D

“…The T e gradient across the x-point is larger than the one across the O-point as shown in figure 2(a), which causes the thermal transport to lead the change in theñ/n resulting in the positive (CCW) hysteresis. This result is consistent with the previous result that theñ/n level increases as the T e gradient is increased [10]. However, the δτ in the O-point is positive in the inner region (ρ < 0.68) and negative in the outer region (ρ > 0.68).…”

“…The magnitude of theñ/n at the O-point is smaller than that at the X-point by a factor of 2 ∼ 3, while the magnitude of theñ/n outside magnetic island region ( ρ = 0.82-1.0) is almost identical between the O-point phase and the X-point phase. These results are consistent with the previous results observed in the Doppler back scattering (DBS) [9,10] and a significant reduction of thermal diffusivity inside the magnetic island observed in JT-60U [11]. The lower level of theñ/n in the O-point seen in Figure 2(f) results in the reduction of transport, which is consistent with the slower heat pulse propagation as seen in Figure 2(e).…”

Magnetic-island-induced ion temperature gradient mode: Landau damping, equilibrium magnetic shear and pressure flattening effects

“…Further growth of long-wavelength magnetic islands as well as their interaction and overlap can lead to very dangerous consequences including the major disruption [22][23][24][25]. In recent experimental studies, the interactions between magnetic island and turbulence have also been widely investigated [26][27][28][29][30]. In advanced tokamaks such as ITER, particularly, so-called neoclassical tearing mode [31] is the most dangerous magnetohydrodynamics instability since it is believed to be a very important reason for triggering disruptions.…”

Development of ECE/ECEI diagnostics and MHD-related studies on HL-2A tokamak

“…The X-point and O-point of the island are indicated with the black X and O marks in the figures 7(a) and 7(b), respectively. Recently, multi-scale physics such as the interaction between magnetic islands and turbulence was found to play an important role in regulating the turbulent transport [15,[17][18][19][20][21][22][23], and therefore has become an important topic in fusion research. ECEI was demonstrated to be a powerful tool for analyzing such multi-scale physics.…”